[sdiy] (easy) resonant filter

[Tim Escobedo]

Go to http://www.planetee.com/ and search the Ideas for Design articles
(you'll have to register for this one) for the April 30, 2001 article,
"Second Order Audio Filter Performs Multiple Functions". It's a rundown
on a nice state variable filter design using inverting op amps and
describes how to independently adjust gain, cutoff, Q, with various
responses.

The simplest resonant filter I know of is a bridged T or twin T using a
single opamp (or even transistor). They can sound pretty good even
though they seem to commonly be implemented with fixed resonance. These
can be easily put under voltage control too. Unfortunately, i don't
know of a really good online resource describing the ins and outs of
this design. There is an article in PAIA site describing a "drum
oscillator/filter" that is a basic voltage controllable twin T using a
single op amp. 

--- Tavys Ashcroft <bigtex at cwnet.com> wrote:
> >can anyone pont me towards the worlds simplest resonant filter
> schematic-i'm
> >bored an want to build something, doesn't have to be voltage
> controllable,
> >but no CEM chips (preferably, just diodes, and simple op amps).
> nothing
> >fancy. or anything else realy simple and fun.
>
> Actually, that's exactly what I'm looking for too.  I just bought an
> unbelievable number of opamps (400 I think) from BG Micro for about
> $8, and want to put them to use (yes, they're 741s, but I'm not
> worried about it, I'm learning).  They're quad opamp chips, so I
> figure 1 opamp for the input buffer and 3 for the filter, giving me a
>
> 1 chip filter.  Then I could make lots of them and have them really
> tiny and give them out as party favors or something.  Is there an
> easy tuneable filter (HP,LP, or BP, resonant or not) that can be made
>
> with three opamps?  I also don't care about voltage control,
> linearity, a perfectly clean signal, or anything really.  I just want
>
> to experiment with filter design and I need a starting place.  I've
> looked around and all the schematics for filters I find require some
> sort of CEM chip.
>
> -Tavys
>


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